1Department of Anthropology, University of Minnesota, 2Department of Ecology, Evolution and Behavior, University of Minnesota, 3Department of Primatology, Max Planck Institute for Evolutionary Anthropology, 4Primate Research Institute, Kyoto University, 5Department of Evolutionary Anthropology, Duke University, 6Wildlife Research Center, Kyoto University, 7Department of Anthropology, University of Michigan, 8Gombe Stream Research Centre, the Jane Goodall Institute, 9The Lester E. Fisher Center for the Study and Conservation of Apes, Lincoln Park Zoo, 10Department of Anthropology, Iowa State University, 11Department of Anthropology, Washington University in St. Louis, 12Department of Anthropology, University of Oregon, 13Department of Anthropology, Yale University, 14School of Psycholgy, University of St. Andrews, 15Department of Human Evolutionary Biology, Harvard University

Wild chimpanzees sometimes kill other chimpanzees. Why they do so has been controversial. Here we use a species-wide dataset to ask to what extent such killings (a) occur in the species as a whole, (b) result from human disturbances, such as deforestation, hunting, or food-provisioning, and (c) occur frequently enough to affect behavior. Including only cases in which the attack was observed, the body was found, or observers found other compelling circumstantial evidence, data from 17 habituated communities at 10 sites revealed 77 cases of killings by chimpanzees. Most killings (78%) were conducted by groups of males, and most victims (82%) were also male. More victims were infants (57%) than adults (35%); juveniles and adolescents were rarely targeted. Most killings (68%) involved intergroup attacks. The number of killings recorded per site was related to the number of males in a community, but not to measures of human disturbance. Expressed in terms used for homicide rates, males killed other grown males at a median rate of 4,658 per annum per million individuals. In contrast to chimpanzees, no lethal conspecific aggression has been documented among wild bonobos (N = 4 communities at 3 sites). We conclude that lethal aggression is a species typical behavior of chimpanzees that occurs sufficiently often to affect the evolution of chimpanzee behavior.

This study was funded by National Science Foundation grant BCS-0648481. Numerous additional sources of funding have supported the long-term studies that contributed data to this study.